The present invention relates to an image forming apparatus, or so-called copying machine or printing machine, which electrophotographically produces an image on paper.
In the field of image forming, as the data volume to be processed has continued to increase, the demand for high-speed printing of high-quality, fine images is increasing year by year.
In reproducing high-quality, fine picture images by the known process of electrophotography, which involves all electrophotographic processes, including a charging process, an exposing process, a developing process, a transferring process, an erasing process, a cleaning process and a fixing process, a technique for efficiently transferring a toner image produced on an image retaining body (photosensitive body) onto a transfer material (paper is mainly used) is important.
The transfer means employed in the transferring process is briefly classified into a type using a corona discharge device and a type using a transfer roller.
Where a corona discharge device is used, the corona discharge device is installed at a specific distance away from the surface of the photosensitive body. As an electric charge of reversed polarity to that of the toner retained on the photosensitive body is supplied to the back of the paper (the surface not in contact with the surface of the photosensitive body) by the corona discharge device, the toner on the photosensitive body is transferred and adsorbed onto the paper by electrostatic attraction.
Where a transfer roller is used, the roller is positioned in contact with the photosensitive body. As a transfer bias is applied to the transfer roller, the toner retained on the photosensitive body is transferred onto the paper that is fed between the photosensitive body and transfer roller.
The corona discharge method is regarded as more effective for high-speed printing than the transfer roller method because the paper can be electrically charged at a higher speed. However, the corona discharge method is disadvantageous in that the transfer efficiency is easily affected by a change in the dielectric constant of the paper dependent upon a change in the paper thickness and/or ambient humidity and also by the thickness of the toner image to be transferred from the photosensitive body.
The transfer roller method is advantageous in that the method is applicable to printing on a variety of types of paper having different specifications because the electric charge is applied directly onto the paper that is in close contact with the toner image on the photosensitive body. However, because the transfer area is a relatively narrow nip zone formed by the contact between the photosensitive body and transfer roller, and because the paper passes through the nip area very quickly under a high-speed printing condition, a sufficient transfer time cannot be maintained. Hence, as is understood, it is difficult to apply the method to high-speed printing. Although this problem can be eliminated by installing more transfer rollers to widen the nip zone, use of more transfer rollers leads to an undesirable cost increase.
An apparatus proposed in Japanese Patent Application Laid-open Publication No. Hei 7-319331 (1995) has a composition such that the angle of the tension adjusting plate installed before and after the transfer unit is set in accordance with the ream weight of the paper to be used. However, the technique used in this Japanese Patent Application Laid Open Publication No. Hei 7-319331 (1995) is based on the need to constantly adjust the tension and length of the paper wound onto the photosensitive body by means of the above-mentioned composition, and does not suggest any technical idea for changing the length of paper wound onto the surface of the photosensitive body.